Appendix VI. 1025 under all circumstances, mitre wheels r and ^ engage through the medium of a face wheel /, and the motion is transmitted to the tool spindle through the spur gear v. The interposition of the face wheel permits therefore a connection of r and s, whatever the angular relation of a and b. CHAPTER VI. P. 193. Relieving or Backing-off Lathe:—In the description of the making of screw-taps, p. 192, the clearance or backing-off was said to be done by hand, but this is by no means the best or proper method of doing it. - Taps are now relieved or backed-off by the aid of some machine appliance, which gives the turning or screwing tool such a transverse motion as will cause it to cut deeper into the tap at certain times, as shewn in section at Fig. 203, p. 194, the flutes having been previously cut as at A, Fig. 185. Relieving lathes, specially built for backing-off while turning, have a much wider and more important field of applica- tion, however, in the making of milling cutters of every con- ceivable form, and the machine shewn in Figs. 934, 934^, and 93 4^, built by Mr. J. E. Reinecker, and introduced in England by Messrs. Pfeil & Co., from whom the drawings have been obtained, will now be described. The general view of the lathe is given in Fig. 934, the lower diagram being a section through the bed, so as to shew the arrangement of the shafts. The driving is as usual by cone pulley A and back gear, and the milling cutter to be formed is mounted on a bar and placed between the cone centres so as to rotate at a suitable speed for the cutting. The cutter blank has been previously roughed out in the manner shewn at ?n, where the deep radial grooves indicate the number of teeth to be adopted. These grooves are generally of spiral form, having long pitch, so can easily be* done in the ordinary milling machirte between centres that are slowly rotated by what is called a spiral attachment. The object of the relieving lathe is next to give each tooth of the cutter m its proper clearance angle by a process of turning, the material being removed down to the dotted lines.